1. Field of the Invention
This invention relates to a reflective-type photoelectric switch, a Fresnel lens capable of being used effectively in the reflective-type photoelectric switch, and a method of manufacturing the lens.
2. Description of Related Art
FIG. 24 illustrates a portion of a conventional reflective-type photoelectric switch. This reflective-type photoelectric switch is equipped with a light-emitting element 1, a light-projecting convex lens 101 for collimating light emitted by the light-emitting element 1 and projecting the collimated light toward a detection zone, a light-receiving convex lens 102 for condensing light reflected from an object to be sensed located in the detection zone, and a light-receiving element 2 for receiving the light, which is condensed by the light-receiving lens 102, and outputting a signal indicative of the light received. The switch is adapted to output a signal, which indicates detection of the object, based on the output signal of the light-receiving element 2. The light-emitting element 1 and light-receiving element 2 are optically shielded from each other by a barrier.
The conventional reflective-type photoelectric switch employs the convex lenses as an optical system for receiving light. Accordingly, though the object to be sensed can be sensed if it is at a distance remote from the photoelectric switch, it cannot be sensed if it is in close proximity to the switch.
The curve represented by the dashed line indicated at A in FIG. 7 illustrates the sensitivity of the conventional reflective-type electromagnetic switch. The horizontal axis in FIG. 7 represents the distance from the photoelectric switch to the object to be sensed, and the vertical axis represents the level (received-light output) of the output signal from the light-receiving element 2. The object is capable of being sensed within a range in which the received light output exceeds a threshold value. The received light output of the conventional photoelectric switch indicated by the curve A declines sharply very near the switch, and the range of distances very near the switch is a blind zone in which the object cannot be sensed.
A structure with a lens arrangement so adapted as to make possible detection of an object not only in a remote region but also at very short distances is disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 59-125709. In the structure having this arrangement, the light-projecting lens and light-receiving lens are both constituted by convex lenses, and a concave lens is provided between the light-projecting convex lens and the light-receiving convex lens so as to correspond to a light-emitting element, and the concave lens is arranged in proximity to the light-projecting convex lens or is integrated with the light-projecting convex lens. A portion of the light emitted by the light-emitting element is collimated by the light-projecting convex lens and is projected off into the distance. The remaining portion of the light emitted by the light-emitting element is diverged to irradiate the range of short distances by the concave lens. As a result, the projected light and the range of detection are widened over a range of from very short to long distances.
With the structure having the foregoing lens arrangement, however, the concave lens is used in order to irradiate the range of short distances. Consequently, the projected light is diverged equally in all directions and therefore the intensity of the projected light is low at very short distances. As a result, there are cases where received-light output of the light-receiving element cannot attain a level high enough to enable detection of the object. Detection at very short distances becomes unstable particularly with regard to an object exhibiting low reflectance. Furthermore, since the projected light is diverged by the concave lens, as mentioned above, the detection zone is broadened and an object at a specific position cannot be detected. Thus, various problems arise.